In reality, a CFD has never been trusted blindly. It has been, all the time, compared to experimental studies and this showed, in general, good agreements between them.
So, my advice, is to set some simulations according to your problem type, and if this gives good results compared to your calculations and/or experimental data, you can use these type of simulation.

To summarize, you can rely on simulation results ONLY after being extensively assessed for a particular type of problem.

You need to know what inputs to use. FLUENT is very accurate program but only if you use good solver configuration to avoid "shit in - shit out". But you should look on some verification experiments for similar cases.